1. Field of the Invention
The present invention relates to a method and apparatus for producing and shipping hydrocarbons, e.g., crude oil, from an offshore site. In particular, the present invention relates to a method and apparatus which does not require an offshore processing plant and which allows both gas and oil to be shipped to an onshore processing plant.
2. Description of the Prior Art
Crude oil and natural gas from offshore wells is produced in the following manner according to the teachings of the presently-known prior art technology. First, the crude oil and gas wells are drilled and completed using drilling equipment that is mounted on either a jack-up drilling rig or on a floating vessel.
After the wells have been drilled and completed they are typically connected to an offshore processing plant that separates the live crude oil from the well--which is typically a mixture of oil, gas, water, salt and other solids--into a stabilized crude oil with a low vapor pressure--that is therefore suitable for transportation in ordinary tanker vessels--and a natural gas component--that is suitable for transportation onshore by a pipeline. Ordinarily, the stabilized crude oil is processed at the offshore processing plant sufficiently so that it may be used in a standard onshore refining process without further treatment to remove solids, salt, and water from the crude oil. Therefore, the offshore processing facility also removes water, salt and other solids from the live crude oil before it is transferred to the vessel as stabilized crude oil.
The stabilized crude oil may then be transported ashore by pipeline or by tanker vessels, which tanker vessels normally store the stabilized crude oil at or near atmospheric pressure. The produced gas is ordinarily transported ashore in pipelines. In addition to transporting the produced gas ashore by pipeline, a number of emerging technologies exist to transport the gas in ships, by subjecting the gas to chemical processes that convert it, for example, into methanol or by liquefying the gas and transporting it as a cooled liquid. The technologies for transporting the gas in ships all require large capital expenditures and cause the loss of a significant fraction of the energy content in the gas during processing and transportation.
If tanker transportation of the stabilized crude oil is used from the offshore oil field processing plant, significant hydrocarbon losses usually occur due to de-gassing of the crude oil in the cargo tanks. The economics and safety of ordinary tanker transportation do not permit the re-capture and retention of this gas, leading to the waste of this energy source.
In the event that no pipeline is available to transport the gas ashore, because of, e.g., distance, many jurisdictions today require that the gas be re-injected into the hydrocarbon-bearing soil formation to preserve the gas for future production when the economics of exploitation permits the production and transportation of the gas. At locations where re-injection requirements do not exist, the gas may be burned in a flare. Either of these processes, re-injection or flaring, are expensive and waste energy that could otherwise be produced or used.
The offshore processing plant of the presently-known prior art technology may be mounted on a platform sitting on the sea bed, on a ship-like vessel, on a semi-submersible, or on a tension leg platform. Other possible means of mounting offshore processing plants also exist. However, all of these means have in common the fact that the platform for supporting the processing plant is very expensive.
The offshore processing plant of the presently-known prior art technology is expensive compared to a comparable crude oil processing plant on land, because the offshore processing plant must be specially adapted for the offshore environment, for operation in a restricted space, to compensate for possible movement and accelerations of the plant during operations, and for limited possibilities for maintenance. Furthermore, the crew operating the offshore plant is regularly ferried back and forth between the platform and land, and all their needs, with the possible exception of fuel, must also be ferried to the plant from shore.
Thus, the capital costs and the operating costs for an offshore processing plant of the presently-known technology is much higher than for a corresponding plant on land.
Some of the problems of the above-described method are addressed in U.S. Pat. No. 4,446,804. In this patent, a method is described for loading shuttle ships with live crude oil directly from subsea oil wells. This process consists of loading the live crude oil into tanks on the shuttle tanker that are pre-filled with a displacement liquid and pressurized to a pressure near the pressure of the live crude oil to be received. The live crude oil then displaces the displacement liquid under nearly constant pressure during the loading operation. This procedure results in a shuttle tanker having an extraordinary complex cargo handling system with a large number of valves and instruments. Another disadvantage of the system described in U.S. Pat. No. 4,446,804 is that the tanker must be designed for a pressure near the bubble point of the crude oil, to take full advantage of the shuttle tanker loading system.
The system described in U.S. Pat. No. 4,446,804, however, has the advantage of minimizing the release of gas from the crude oil by maintaining the cargo always near maximum pressure. A severe drawback to the system described in U.S. Pat. No. 4,446,804 is that the containment system in the tanker must be designed for the bubble pressure of the received crude oil. This pressure varies from oil field to oil field. Therefore a tanker may be designed to serve a specific oil field, which limits its utility, or may be designed to be used in a number of oil fields. In the latter case the cargo containment system must be designed for a highest pressure in the oil fields, possibly as high as 35 MPa.
Another relevant patent to this field is U.S. Pat. No. 5,199,266. This patent describes a method for transporting gas from offshore fields, which gas has been produced on offshore production platforms by pressurizing the gas and cooling it to a temperature in the range of -100.degree. C. to -120.degree. C. In this temperature range and at a pressure of approximately 1.5 MPa, all hydrocarbon gases normally occurring in oil wells are liquid. As described in U.S. Pat. No. 5,199,266 the gas must be delivered to the transport vessel in gaseous form and is then cooled and liquefied on the shuttle vessel. A very large and expensive cooling plant is required on the gas transport vessel to cool and condense the gas to be transported. Thus, the system described in U.S. Pat. No. 5,199,266 not only requires an offshore production platform in accordance with the traditional technology but also require a number of high pressure, refrigerated tanker vessels each fitted with a large-capacity cooling plant.